5.29.2007

This is the most dangerous naturally occurring toxin: the Botulinum toxin.

It is the trademark of Clostridium botulinum bacterium and less than 2 kg (5 pounds) would be enough to wipe out the whole human population. Even so, in extremely small amounts, this toxin has medical and cosmetic employments, under the name of Botox.

Now the genome of this deadly bacterium has been decoded. It appears that C. botulinum doesn't possess methods to cope with the human immune system or the antibiotics. The bacterium lives as a dormant spore or as an active scavenger of decaying soil animal matter, and cannot interact with human or other large animal hosts for longer periods of time.

When the bacteria manage to enter a living animal, through contaminated food or open wounds, it triggers botulism, a severe infection that can easily kill children. If the bacteria manage to kill the host with their toxin, they get a new food source. "Although in the same group as Clostridium difficile - the Cdiff superbug - C. botulinum has a genome that is remarkable because it is so stable. Unlike Cdiff, in which more than 10% of genes have been acquired from other bacteria, there is almost no footprint of these in C. botulinum.", commented lead author Dr Mohammed Sebaihia.

Several strains of C. botulinum are known, and even if they are described as a unique species, they are very different bugs with just one thing in common: the botulinum toxin.

And each strain has a harmless relative missing the toxin, grouped as C. sporogenes. "43% of the predicted genes in the C. botulinum genome are absent from the other five sequenced clostridia, and only 16% of the C. botulinum genes are common to all five.", explained Professor Mike Peck, from the Institute of Food Research.

Botulinum toxin is neurotoxic: it stops nerves, inducing death (this is also the mechanism by means of which the wrinkles' muscles relax).

Over 110 of the bacterium's 3700 genes are involved in controlling spore formation and germination, enabling the bacterium to hibernate and attack when opportunity comes along. "Some organisms use subtle approaches, elegantly choreographing their interaction with us and our defenses. C. botulinum takes the opposite approach. It lies in wait and, if it gets the opportunity, it hits its host with a microbial sledgehammer. It then eats the remains and lays low until the next host comes along.", explained Dr Julian Parkhill of the Wellcome Trust Sanger Institute.

This bacterium was also found to possess unique genes that allow it to digest the chitin exoskeleton of the insects and other arthropods (like spiders and small crustaceans) encountered in the soil. "It is not only animals that can feel the wrath of C. botulinum. The soil can be a harsh environment and food can be scarce. To see off the competition, C. botulinum comes with its own 'antibiotic' - a chemical called boticin that kills competing bacteria.", explained Sebaihia.

"We have developed a highly efficient system, the ClosTron, with which we have, in a few months, knocked out over 30 genes in four different clostridial species, including eight in C. botulinum. The availability of this tool should revolutionize functional genomic studies in clostridia.", said Nigel Minton, Professor of Applied Molecular Microbiology at the University of Nottingham, the developer of this method.